Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C4/00—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge
  • C23C4/04—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge characterised by the coating material
  • C23C4/10—Oxides, borides, carbides, nitrides or silicides; Mixtures thereof
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS, OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
  • F27D1/00—Casings; Linings; Walls; Roofs
  • F27D1/16—Making or repairing linings increasing the durability of linings or breaking away linings
  • F27D1/1636—Repairing linings by projecting or spraying refractory materials on the lining
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS, OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
  • F27D99/00—Subject matter not provided for in other groups of this subclass
  • F27D99/0001—Heating elements or systems
  • F27D99/0006—Electric heating elements or system
  • F27D2099/0031—Plasma-torch heating
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/25—Process efficiency

Definitions

• the present invention relates to a method for producing a coating with a titanium boride content of at least 80% by weight, with a thickness of 0.1 mm to 1 mm, with a porosity of at most 10% by volume and with an oxygen content of less 1 wt .-% by plasma spraying.
• the cathode-side power supply takes place through the carbon blocks, which are connected to one another by temperature and corrosion-resistant adhesive or ramming compounds and are enclosed by a metal trough or a container.
• the electrical current is supplied to the carbon blocks via busbars or bars which are embedded in recesses in the carbon blocks and are connected to them.
• Anodes which are attached to the anode bar and consist essentially of carbon in conventional processes are immersed in the electrolyte from above. Oxygen is generated at the anodes by the electrolytic decomposition of the aluminum oxide, which converts to carbon dioxide and carbon monoxide in the case of carbon anodes.
• the electrolysis generally takes place in a temperature range between 940 and 970 ° C.
• a major disadvantage of the cathode blocks made of carbon is their poor wettability due to the molten aluminum formed during the operation of the electrolysis cell. For this reason, a comparatively thick aluminum metal layer covering the carbon blocks is required for the operation of the cell necessary. Since thick metal layers are deformed considerably by electromagnetic forces and convection currents, a comparatively large distance between the carbon blocks and the carbon anodes arranged above the blocks must be maintained in order to avoid a possible short circuit. This leads to a higher electrical power requirement of the cell. Furthermore, the metal flow generated at the liquid aluminum / electrolyte phase boundary leads to an increased chemical dissolution or to a fine dispersion of the aluminum in the electrolyte due to the low interfacial tension. Any dispersed aluminum that comes close to the anode will be in contact with the anodized carbon monoxide and
• the ideal cathode material should also meet further requirements: it must have sufficient mechanical strength, be resistant to thermal shock, be sufficiently electrically conductive and have sufficient adhesion to the have underlying cathode blocks, if it is a Coating is concerned.
• Titanium diboride is a very brittle material that is sensitive to thermal shock and has poor resistance to impacts or shocks.
• US-A-4 466 995 discloses a method for coating the cathode surface with a mixture of refractory hard material (RHM), thermosetting binder, solvent and carbon-containing filler material, where RHM stands for the compounds TiB 2 , TiC, ZrB 2 or ZrC .
• RHM refractory hard material
• EP-A-0 021 850 describes the electrolytic deposition of TiB 2 from a molten electrolyte as a coating method, which contains titanium dioxide or a titanate or a borate as a source of titanium or boron.
• DE-A-23 05 281 describes a process for the production of cathodes, in which a coating or a coating of molten or highly sintered, dense, refractory hard material is applied to a surface.
• the hard materials are the borides, nitrides, carbides and silicides of transition metals in the fourth to sixth groups of the periodic table.
• This melt layer can be obtained either by heating to temperatures of 2200 to 2300 ° C or by plasma spraying.
• DE-A-23 12 439 describes a method for coating a container for a cathode tub.
• a thin coating of electrically conductive, ceramic material is applied to this container by introducing the ceramic material into an ionized gas jet with high energy content and applying the material in a molten state.
• the ionized gas jet is surrounded with a protective jacket ("shroud") made of inert or reducing gas.
• titanium diboride has not yet been used commercially as an electrode material in the aluminum industry.
• the lack of acceptance is due to the short lifespan in cell operation. It has been reported that such current-carrying elements after a very short time
• Titanium diboride tiles for example, are susceptible to rapid attack by aluminum and / or cryolite melt at the grain boundaries, since the oxygen contaminants preferentially accumulate at the grain boundaries during the sintering process.
• Previous attempts to prevent titanium diboride tiles from dissolving led to the use of high purity titanium diboride powder. The oxygen content was less than 50 ppm and the price was three to four times higher than for normal powder with an oxygen content of about 3000 ppm.
• Another disadvantage is that the production costs for the tiles increase considerably because special sintering conditions must be observed.
• Cathodes or cathode coatings made of a composite material of titanium diboride with carbon, graphite and / or titanium carbide or similar hard materials cannot be wetted so well by aluminum, so that no great effect in terms of energy saving can be expected here.
• the additives worsen the electrical conductivity, and the mixed material usually also reduces the durability, since it is usually not very resistant to aluminum.
• Thin Solid Films, 224, 1993, 148-152 teaches that titanium diboride coatings made by atmospheric plasma spraying can contain up to 14% by weight of oxygen due to their manufacturing process, with the starting powder containing 2% by weight of oxygen Has. These coatings are therefore very susceptible to corrosion from liquid aluminum.
• the use of a gas sheath (“shroud") around the plasma torch only inadequately reduces the oxygen absorption by the reactive titanium diboride to 9% by weight.
• Inert gas plasma spraying in which both the body to be coated and the plasma torch are completely in an inert gas atmosphere, avoids the introduction of oxygen, but gas inclusions in the coating lead to a quite porous coating, which has a very large area of attack which offers corrosion and erosion.
• the invention had for its object to provide a method for producing a titanium boride-containing coating for large-area support bodies, in particular for carbon and / or graphite bodies, with the corrosion and erosion-resistant, firmly adhering coatings that can be produced for use in a Aluminum melt flow electrolysis cell are suitable. Another task was to propose an economically working process that can be used for an industrial process.
• the object is achieved with a method for producing a coated
• Support body in which the coating has a titanium boride content of at least 80% by weight which is characterized in that a coating of 0.1 mm to 1 mm thick with a porosity of at most 10% by volume and with an oxygen content of less than 1% by weight is applied to the surface of a carrier body by plasma spraying in an atmosphere which is almost or completely free of oxygen, no metallic powder being added to the wettable powder.
• its titanium boride content must be at least 80% by weight and its thickness must be at least 0.1 mm.
• the relatively large layer thickness is because of Surface profiling due to the porosity and coarse structure of the carrier body material is necessary.
• the maximum layer thickness should therefore be at most 1 mm, the porosity at most 10% by volume. Coatings with an average thickness between 0.2 and 0.5 mm are particularly favorable. A porosity of 4 to 6% by volume is preferred, since the pores do not allow a connection between the carrier body and the medium even with relatively small layer thicknesses, but the mechanical stresses in the coating are sufficiently low due to the residual porosity present.
• Titanium boride coatings are obtained with a significantly higher purity than is the case with conventional sintering to titanium boride bodies or coatings.
• the method according to the invention is suitable for coating carrier bodies of any shape, preferably carrier bodies made of carbon and / or graphite.
• Such bodies are in particular bodies made of synthetically produced carbon or graphite, which have been obtained from a pre-product body consisting of a filler mixture and a binder by treatment at high temperatures with the exclusion of substances having an oxidizing effect; this means either bodies that have been heated to temperatures of not more than 1400 ° C.
• the filler content in addition to a portion of non-graphitized coke contains particulate graphite.
• They are preferably large flat carrier bodies, so that areas of more than 1 m 2 can be coated.
• the carrier body surfaces can e.g. prepared for coating by sandblasting, arc cleaning and / or heating up to about 600 ° C.
• the carrier bodies must be largely free of grease, dust and dry.
• powder delivery rates of at least 30 g of powder per minute, in particular of 40 to 60 g of powder per minute, must be used.
• the spray powder used must meet a corresponding specification. This means particularly high demands on the particle sizes or particle size distribution of the wettable powder or wettable powder mixture. Good pourability and uniformity are assumed in this application for a wettable powder.
• the average grain size of the wettable powder or wettable powder mixture should not be more than 55 ⁇ m, since with larger grain sizes above this value the coarser powder particles do not melt sufficiently due to the short residence time in the plasma flame and the high melting point of titanium diboride. This leads to the inclusion of round, not completely melted particles in the coating and consequently to mechanical stresses within the coating.
• a wettable powder or wettable powder mixture with a fine fraction less than or equal to 3 ⁇ m of less than or equal to 1% by weight is melted, since small powder particles melt very quickly compared to the large powder particles, overheat and partly evaporate. This can lead to deposits on the wall of the plasma nozzle and thus clog the nozzle.
• the long coating times from 1 to 4
• a somewhat coarser wettable powder or wettable powder mixture is advantageous for the coating process, which results in increased process reliability and also has a lower oxygen content. Due to the larger specific surface area of a finer powder, significantly more oxygen is introduced. As an example shows a TiB 2 powder fraction with ad 50 value of 12 ⁇ m im
• the oxygen content was twice as high. This means that it is not absolutely necessary to use a powder which is less oxygenated due to a particularly complex manufacturing process, but the use of a coarser grain fraction can reduce both the oxygen content and the costs sufficiently. Surprisingly, it was found that application under a slight negative pressure leads to a reduction in the oxygen content in the coating compared to the starting memever used. This process works particularly effectively when a certain residual carbon content is present in the starting powder, which enables the formation of volatile compounds in the form of CO or carbonyl compounds.
• the oxygen content in the wettable powder or wettable powder mixture which is melted is preferably less than or equal to 1% by weight. If the oxygen content is above 1% by weight, an increased carbon content should also occur in proportion to it.
• the ratio of oxygen content to carbon content in the wettable powder or wettable powder mixture is then preferably in the range from 0.7: 1 to 5: 1.
• the term wettable powder is also understood to mean a technical quality which, due to the production process, has a small content of various impurities.
• Typical impurities in addition to the main impurities oxygen and carbon already mentioned are, for example, nitrogen, iron and other metallic impurities. The sum of these contents is usually less than 2.5% by weight, often less than 1% by weight.
• no metallic powder is added to the wettable powder.
• the metallic impurities are preferably ⁇ 1% by weight, particularly preferably ⁇ 0.5% by weight, very particularly preferably ⁇ 0.1% by weight.
• compositions of the wettable powders or wettable powder mixtures for this invention are not explicitly mentioned; rather, is from calculated from a 100% pure material.
• a coating with a content of metallic impurities of ⁇ 1% by weight, in particular ⁇ 0.5% by weight, is preferably produced.
• the application efficiency (quotient of the amount of powder applied as a coating to the amount of powder conveyed) decreases with decreasing atmospheric pressure in the coating chamber. At pressures below 500 mbar, the application efficiency even goes to zero. Reducing the pressure in the coating chamber can therefore significantly increase production costs, since the starting powder is a major cost factor.
• the method can be optimized so that the porosity of the coating is suitable for the application, the adhesion to the surface is sufficient and the
• Plasma is preferably sprayed in an almost or completely oxygen-free atmosphere at a pressure of at least 500 mbar, in particular from 750 to 950 mbar.
• a mixture of essentially argon and hydrogen is used in particular as the plasma gas.
• the plasma flame is preferably operated with a plasma gas mixture with 60 to 80% by volume argon and 40 to 20% by volume hydrogen. With this composition, the necessary energy for the high-melting titanium boride particles is introduced and a sufficiently good melting of the powder particles is achieved.
• the adhesion of the titanium boride coating according to the invention to the carrier body made of carbon or / and graphite is so high that when the coating is stressed e.g. a crack which occurs as a result of impact or impact, insofar as it runs essentially parallel to the coating surface, does not run through the middle of the coating and not through the interface between coating and carrier body, but in the carrier body.
• the uniformity of the structure is ensured due to the uniformity of the wettable powder and the process conditions.
• the wettable powder can be mixed homogeneously with 0 to 20% by weight of zirconium boride powder, preferably with a particle size distribution similar to that of the titanium boride wettable powder, and processed in the same way as that.
• Spray powder mixture preferably in the preferred particle size distribution and in an amount of up to 12% by weight, particularly preferably up to 6% by weight, very particularly preferably up to 2% by weight.
• the addition of these additives is not limited by the process, but by the further properties of the coating for the intended use, such as, for example, by the chemical resistance to liquid aluminum and the Electrolyte melt at operating temperatures.
• the proportion of titanium boride is preferably 84 to 99.99% by weight, in particular 88 to 99.95% by weight, particularly preferably more than 94 or 99.9% by weight, especially 97 to 99.85% by weight .-%, primarily about 99.8 wt .-%.
• the body is primarily used to coat carrier bodies made of carbon and / or graphite, in particular partially or entirely graphitized carbon bodies.
• a carbon body is preferably coated as the carrier body, which was heated to temperatures of not more than 1400 ° C. during production, which was heated to temperatures in the range from 1600 to 3000 ° C. during production, or which was not heated to temperatures during production more than 1400 ° C and one binder coke portion and one filler coke portion made of non-graphitized coke as well as a further filler portion that is at least 30
• Wt .-% consists of particulate electrographite, contains.
• support bodies made of other materials such as steel, other metals or alloys and ceramics, in particular refractory metals and hard materials, and also composite materials can also be coated.
• the coated carrier bodies can also be used for other purposes such as e.g. can be used according to the invention as an electrode, in particular as a cathode element, as a heating element, as a refractory lining, as a heat shield, as a wear-resistant element or as a container, in particular as a crucible or evaporator boat, as a nozzle, as an element of a heat exchanger or nuclear reactor.
• a feature of the method is that the carrier body can be plasma-coated in only one or two passes.
• the entire thickness of the coating is preferably applied in one passage and the entire
• a single operation leads to a lower oxygen content of the coating and to shorter coating times.
• an intermediate layer can be applied between the carrier body and the coating to adapt the different coefficients of thermal expansion before the plasma coating with spray powder or spray powder mixture containing titanium boride.
• the process parameters are preferably chosen so that this intermediate layer can be saved for cost reasons.
• Intermediate layer can consist of the same or a chemically very similar material as the protective coating and possibly with a higher porosity or of another material with a suitable expansion coefficient.
• boride, carbide, nitride and / or silicide of aluminum or / and metals of the 4th, 5th and 6th group of the periodic table Ti - Cr, Zr - Mo, Hf - W
• boride content is particularly preferably up to 12% by weight, very particularly preferably up to 8% by weight, and the contents of carbides, nitrides or / and silicides of each of these three classes of compounds are up to 6% by weight. very particularly preferably up to 3% by weight.
• the method according to the invention is suitable for the joint-free coating of arbitrarily shaped carrier bodies. It can be used to coat electrodes and other large-area carbon and graphite bodies. Especially it is advantageously suitable for coating large carbon cathode elements for aluminum melt flow electrolysis.
• the individual cathode elements usually have a length of up to 4000 mm and a width of up to 800 mm. These surfaces can be coated in one piece without interruption.
• the height of a cathode element is usually 400 to 500 mm, the weight of a cathode element is up to 2.51.
• the carbon or graphite bodies provided as carrier bodies were stored dry and dust-free after their manufacture, surface treatment by milling and cleaning and were accordingly introduced into a vacuum coating chamber for coating.
• the plasma torch was movable in the x, y, z direction.
• the chamber was first evacuated to a residual pressure of about 10 "2 mbar and then flooded with argon (quality 5.0), the atmospheric pressure being increased to 800 to 900 mbar, except for individual tests according to Table I.
• argon quality 5.0
• the plasma gas consisted of an argon-hydrogen mixture.
• a wettable powder or wettable powder mixture which had an average particle size of 10-55 ⁇ m, preferably of The powder particle size distribution of the wettable powders or wettable powder mixtures was measured with a Mastersizer X measuring device from Malvern, special attention being paid to representative sampling and good dispersion with sodium polyphosphate as the dispersant Carbon content was determined using carrier gas extraction with a Me ß device from Leco.
• the wettable powder or wettable powder mixture with the preferred specification - except for individual experiments according to Table II - was blown into the plasma flame via a carrier gas and deposited on the surface of the carrier body, which was about 60 to 100 mm from the plasma nozzle.
• the plasma torch was specially made by Medicoat AG with a burner output of 50 kW, with a nozzle geometry specially developed for the preferred spray powder and process conditions and with an increased cooling capacity. He scanned the surface line by line. The speed of movement and the distance between the individual lines was set so that the desired layer thickness was achieved in preferably one, at most with two passages. Layer thicknesses of 0.1 mm to 1 mm were aimed for.
• the temperature of the carrier body was between 100 and 400 ° C. during plasma spraying. In the case of a longer coating period, care was taken to ensure that the composition of the atmosphere could not change any more by introducing, for example, hydrogen, by pumping off continuously or at certain intervals and circulating through filters to remove the dust which accumulated.
• the electrical resistance was determined on a medium-sized carbon cathode element with a coating according to the invention in accordance with Experiment 8 of Table II by means of two peaks at a distance of 4 cm to 0.5 ⁇ m.
• M x B y stands for a boride of a transition metal of IV., V. and VI.
• Aluminum melt flow electrolysis in particular a composite material made of 98% by weight TiB 2 /2% by weight ZrB 2, also proved its worth because of the reduced brittleness.

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• 1997
  • 1997-04-08 DE DE19714433A patent/DE19714433C2/de not_active Expired - Fee Related
• 1998
  • 1998-03-24 US US09/402,552 patent/US6645568B1/en not_active Expired - Fee Related
  • 1998-03-24 CA CA002285982A patent/CA2285982C/en not_active Expired - Fee Related
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  • 1998-03-24 EP EP98919140A patent/EP0973955B1/de not_active Expired - Lifetime
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